Many different methods have been employed to backwash conventional downflow filters. In most of these methods the backwashing operation is carried out employing clean, filtered wash water in a direction counter to the direction of influent flow during the filtering or service runs.
Probably the most commonly accepted practice for backwashing a granular filter bed in a gravity filter is to supply a fluidizing flow of clean, filtered water in an upflow direction to expand the bed and carry away floc and particulate matter which has been collected in the bed during a previous service run. This washing method is often accompanied by a surface wash with rotary or fixed jets of water. In these prior art techniques it has been recognized that the velocity of the backwashing liquid should be sufficient to expand or fluidize the bed to provide adequate cleaning between service runs.
The prior art also suggests the use of air scouring to augment the cleaning of a filter bed. One such method employs simultaneous air and water flow at rates which are sufficiently high to fluidize the bed. These techniques often result in excessive media loss, resulting from the carrying away of media along with the floc during the backwashing operation.
One prior art technique of utilizing air scouring while minimizing media loss employs a two-stage process. In the first stage air scour alone is applied with no upflow of water in order to agitate and loosen the floc collected in the bed. During the second stage an upflow of water is provided at a sufficiently high rate to fluidize the entire bed, and thereby carry away the loosened floc and debris.
Another prior art technique, which is probably the least commercially applied in the United States, employs the combination of an air scour and a non-fluidizing upflow of water as the sole means to carry out the backwashing operation. Under optimum flow conditions there is an observable formation and collapse of air pockets, and the concomitant local movement of media, yielding maximum particle-to-particle collision and floc disruption with minimum media loss. This condition occurs when the velocity of the air and of the upflow water approximately fulfills the following relation developed by Amirtharajah, et.al.: ##EQU1## wherein "V" is the velocity of backwash liquid; V.sub.mf is the minimum fluidization velocity and Q.sub.a is the air scour rate in standard cubic feet per minute/sq.ft.
This system of employing an air scour and a non-fluidizing flow of water has been used primarily in European conventional downflow filter systems, in which the direction of water flow is reversed for cleaning, and the washing liquid is filtered or clean water. This latter system is operated for a sufficient length of time to completely clean the filter bed, as is evidenced by the clarity of the wash water and the return of the bed to substantially the clean filter bed headloss. Moreover, this approach has been practiced in European conventional downflow filters wherein the filter media is fine (i.e., having an effective size equal to a fraction of a millimeter). In the U.S., this approach has been primarily limited to wastewater applications with coarse media having an effective size greater than one millimeter wherein the incoming influent has a very high solids content and turbidities substantially in excess of 100 nephelometric turbidity units, (NTU). Moreover, in wastewater filtration, the washing liquid is filtered water.
Yet another known method for washing an upflow clarifier or roughing filter is employed in the Microfloc Trident system, as is disclosed in U.S. Pat. No. 4,547,286. This latter system employs a clarifier section that relies upon the use of a filter bed of buoyant (specific gravity less than 1), synthetic media, followed by a conventional downflow filter employing a non-buoyant media bed. The buoyant filter bed is cleaned between service runs by injecting air into the influent to cause the buoyant bed to fluidize and expand downwardly to release floc particles from the bed. Thereafter the injecting of air into the influent can be discontinued, and the continuous upward flow of influent continued to flush out the loosened floc from the bed. Although the '286 patent discloses only partially cleaning the filter bed between service runs, it discloses complete cleaning every fifth cleaning and it accomplishes this result in a bed of relatively expensive, and somewhat difficult to handle synthetic buoyant particles.